Generally, optical communication is divided into a transmitter unit for changing an electric signal into an optical signal, an optical fiber carrying the optical signal, and a receiver unit for changing the optical signal into an electric signal.
In the optical fiber, which is a passive component carrying the optical signal, the optical signal is absorbed by electronic transition of the positive ions of glass in the case of a short wavelength, and the optical signal is absorbed by molecular vibration in the case of a long wavelength. Thus only wavelengths ranging about 1200 nm to 1700 nm may be used for optical communication.
However, only the band of about 1300 nm wavelength and the band of near 1550 nm wavelength are used for the optical communication due to the effects of impurities, such as OH− groups and transition metals contained in the glass which is a preform.
While the 1550 nm band is mainly used over long distance transmission since the transmission loss of silica glass optical fiber is the smallest, the 1300 nm band is mainly used for short wavelength transmission, which is a zero-dispersion wavelength region of a silica glass optical fiber (John B. MacChensey, David J. DiGiovanni, J. Am. Ceram. Soc., 73[12], 1990).
However, the silica glass optical fiber absorbs light as well as transmits light. By this absorption, losses are accumulated and the strength of the signal light decreases exponentially as the transmission distance becomes longer.
To amplify the decreased signal, an amplifier for amplifying the signal should be set at intervals of tens of km in an optical fiber network. At first, for the amplifier of this type, an optical signal is changed into an electric signal and then the signal is amplified. But this method failed in practical use due to the errors generated during an amplification procedure and a time delay to amplify the optical signal.
Therefore, instead of the above-described amplification method, an optical fiber amplifier for amplifying the optical signal itself without conversion to the electric signal has been studied actively.
For the first optical amplifier, an optical amplifier using a barium crown glass as a base material and doped with neodymium was studied (C. J. Koester and E. Snitzer, Appl. Opt., Vol. 3, pp 1182-1186, 1964).
Afterwards, in the late 1980's, erbium doped fiber amplifiers (EDFA) were developed and commercialized, which amplifies wavelengths of the 1550 nm band where the signal loss of a silica optical fiber is the smallest (w. J. Miniscalco, J. Lightwave Technol., Vol. 9, pp234-250, 1991).
However, due to a rapid increase of information transmission quantity, the band width available in conventional wavelength division multiplexing (WDM) techniques has been saturated, thus there was a need for optical amplifiers available in a new band width.
First, using a gain-shifted EDFA, the available wavelength band width was extended to a 1600 nm band which has longer wavelengths than the 1550 nm band. But this band has been also saturated, so there is a need for an optical amplifier in another wavelength band.
Meanwhile, the 1480 nm band adjacent to the 1550 nm band is easily accessible. In the optical amplifier available in the S-band (1430 nm˜1530 nm), an optical fiber for the optical amplifier containing Tm3+ ions is currently being studied and developed.
A fluorescence caused by a transition of Tm3+ from 3H4 to 3F4 level has a central wavelength of 1480 nm and make the optical amplification possible in the S-band.
However, it is difficult to get a 1480 nm fluorescence caused by transition from 3H4 to 3F4 level with a silica glass having lattice vibration energy of about 1100 cm−1 since the interval between 3H4 and 3H5 level (lower level) of Tm3+ is not more than about 4200 cm−1. That is, a thermal transition to 3H5 level (lower level), which is absorbed into lattice vibration of a glass perform, occurs instead of the transition from 3H4 level to 3F4 level.
Therefore, the aim of the current research and development is to get a 1480 nm fluorescence caused by the transition of Tm3+ from 3H4 level to 3F4 level using fluoride glass and sulfide glass with a small amount of lattice vibration energy.
As a result of a recent research, T Kasamatsu published an article (Laser-Diode-Pumped Highly Efficient Gain-Shifted Thulium-Doped Fiber Amplifier Operating in the 1480-1510 nm Band, IEEE Photonics Technology Letters, vol. 13, No. 5, 433-435, 2001) wherein 1480 nm fluorescence is obtained by using a fluoride glass as a base material.
Furthermore, the generation of 1480 nm fluorescence from fluoride glass and its mechanism are explained by Y. B. Shin in an article (Multiphonon and cross relaxation phenomena in Ge—As (or Ga)—S glasses doped with Tm3+, Journal of Non-Crystalline Solids, 208, pp 29-35, 1996).
As another result of the research, U.S. Pat. No. 6,266,181 discloses an optical fiber amplifier using a tellurite glass as a base material. Also, according to an article “Influence of 4f absorption transitions of Dy3+ on the emission spectra of Tm3+-doped tellurite glasses” by Y. G. Choi in Journal of Non-Crystalline Solids, 276, pp 1-7, 2000, 1480 nm fluorescence was obtained.
However, there are many difficulties in connecting with silica-based optical fibers using the fluoride glass, sulfide glass or tellurite glass.
In other words, when the non-silica based optical fiber is connected to the silica-based optical fiber, the quality of signal is degraded incurring the loss of signal due to the difference in their own refractive indexes. Furthermore, conventional fusion splicing procedures cannot be applied because of the difference in softening temperatures of both fibers.
Moreover, since an optical fiber is manufactured by a fusion method, the optical fiber may contain impurities, such as OH− groups and transition metals thereby causing a defect in the optical fiber.
Light loss reaches at about 200 dB/m and the manufactured optical fiber has low chemical endurance due to the penetration of OH− groups. These reasons make it difficult to be put to a practical use.